Field of the Invention
The present invention relates to a non-aqueous electrolyte storage element.
Description of the Related Art
In recent years, accompanied by downsizing and enhanced performance of mobile devices, a non-aqueous electrolyte storage element has improved properties thereof and become widespread. Also, attempts are underway to improve load discharge performance and gravimetric energy density of the non-aqueous electrolyte storage element, aiming to expand applications of the non-aqueous electrolyte storage element to electric vehicles.
Conventionally, a lithium ion non-aqueous electrolyte storage element has been widely used as the non-aqueous electrolyte storage element. The lithium ion non-aqueous electrolyte storage element contains a positive electrode, such as a positive electrode of lithium-cobalt composite oxide, a negative electrode of carbon, and a non-aqueous electrolyte prepared by dissolving a lithium salt in a non-aqueous solvent.
Meanwhile, there is a non-aqueous electrolyte storage element, which is charged and discharged by intercalation and deintercalation of anions in a non-aqueous electrolyte to a positive electrode composed of a material, such as a conductive polymer, and a carbonaceous material, and by intercalation and deintercalation of lithium ions in the non-aqueous electrolyte to a negative electrode composed of a carbonaceous material (the aforementioned type of the battery may be referred to as a “dual carbon battery cell” hereinafter) (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2014-130717).
As indicated by the following reaction formula, the dual carbon battery cell is charged by intercalation of anions, such as PF6−, from the non-aqueous electrolyte to the positive electrode, and intercalation of Li+ from the non-aqueous electrolyte to the negative electrode, and the cell is discharged by deintercalation of anions, such as PF6−, from the positive electrode, and deintercalation of Li+, from the negative electrode to the non-aqueous electrolyte.

The discharge capacity of the dual carbon battery cell is determined with the anion storage capacity of the positive electrode, the anion releasable amount of the positive electrode, the cation storage capacity of the negative electrode, the cation releasable amount of the negative electrode, the anion amount of the non-aqueous electrolyte, and the cation amount of the non-aqueous electrolyte. In order to increase the discharge capacity of the dual carbon battery cell, therefore, it is necessary to increase not only a positive electrode active material and a negative electrode active material, but also an amount of the nonaqueous electrolyte containing a lithium salt (see, for example, Journal of The Electrochemical Society, 147(3) 899-901 (2000)).
A sufficient amount of the electrolyte salt is required in the aforementioned non-aqueous electrolyte storage element, which is charged by accumulating anions into the positive electrode and cations from the negative electrode from the non-aqueous electrolyte, and is discharged by releasing anions from the positive electrode and cations from the negative electrode to the non-aqueous electrolyte. It is important to insert the non-aqueous electrolyte into the limited volume of the non-aqueous electrolyte storage element in order to improve the volume energy density of the storage element. If the porosities of the electrodes are increased to insert a sufficient amount of the non-aqueous electrolyte, however, there is a problem that high load discharge performance is impaired, as contact between active material particles is reduced.
In a non-aqueous electrolyte storage element using a lithium accumulating and releasing positive electrode, such as an oxide complex positive electrode, and a lithium accumulating and releasing negative electrode, such as graphite, a concentration of the electrolyte salt is substantially unchanged with charging and discharging. Therefore, the densities of the electrodes are set high to insert as much an amount of a storing material as possible inside the storage element (to increase the energy density of the storage element), which lowers porosities of the electrodes. In the case where a storage element is composed to have the same structure to that of the storage element where the concentration of the electrolyte salt is substantially unchanged with charging and discharging, an amount of the non-aqueous electrolyte that can be inserted into the storage element is reduced to lower the concentration of the electrolyte salt, leading to a problem that sufficient charging capacity and discharging capacity of the storage element cannot be attained. If the amount of the non-aqueous electrolyte is substantially increased by excessively increasing a thickness of a separator to solve the aforementioned problem, the energy density of the non-aqueous electrolyte storage element is reduced by the increased amount of the separator, which does not contribute to storage of electricity.
In the case where the concentration of the electrolyte salt is increased to about 3 mol/L in a non-aqueous electrolyte storage element using, as a positive electrode, an electrode accumulating therein anions, and the storage element is charged with high voltage, furthermore, there is a problem that a capacity of the storage element is reduced.
Accordingly, there is a demand for a non-aqueous electrolyte storage element, which can realize a high energy density, and a high load discharge performance, and has improved charge-discharge cycle property.